Deluxe Reverb AB763 help, Reverb circuit issue

[Danohss]

So I have an AB763 on the bench, issue with the reverb circuit - with the 7025 (V7, 3rd tube from the end) removed all is good, but with that tube installed, no foot switch or reverb tank connected, amp makes a god awful loud noise with all the volume and tube knobs on zero, like a super loud static.  Voltages all seem to check out.  Tried the obvious - new tube, no change.  Next step will be to disconnect the reverb pot to further try to isolate it.  Anyone ever deal with this issue?
It’s a real 1963, has been fully freshened up - all new electrolytics, bypass caps etc.  Has been working fine for many years since the initial updates, this is a sudden new problem. 

[PRR]

New resistors this century? "Godawful static" reminds me of a tube or a carbon-comp resistor gone rouge.

[pdf64]

I’m confused by the valve references; rather than 7025, V7 is surely a 6V6, 3rd from the end is either a 12AT7 or a 6V6?
Whatever, if the reverb control doesn’t affect the noise, then a noisy anode load resistor on the 2nd or 3rd stage of the fx channel may be likely suspects (V2 pin6, V4 pin6).
If the reverb control does affect the noise level, then the reverb recovery stage anode load resistor (V4 pin1) may be the problem.
Note that resistor noise is unrelated to its conformance to nominal resistance value. ie they could measure close to 100k yet still be bad, noisy.

https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_reverb_ab763_schem.pdf

[Latole]

Unplug those 3 tubes and report.

[Danohss]

Update:  So the amp works fine with either V3 or V4 removed, think I may have a bad choke.  When I look for voltage on the input side of the choke voltage starts up around 1300 VDC then counts down over around 5 seconds to around 430 VDC.  The voltage out of the choke is only at 108, schematic says it should be over 400.  Clearly something in the reverb circuit.  With either tube installed voltages at the 100K resistors to the tubes checks out fine so I assume all 4 of them are good.  They are all carbon comp but have been replaced at least once since 1963.  Figured if one or more of them has gone bad the voltage coming out of those 2 “Y’s” would show a problem.  No voltage showing leaking through any of the caps. 

[PRR]

....voltage on the input side of the choke voltage starts up around 1300 VDC then counts down over ... ...The voltage out of the choke is only at 108...

I am very confused. You had godawful static. But there can't be over a thousand volts in there without a major wiring problem. And a thousand+ volt drop in a low-drop choke??

Where is the smoke coming out? Because if it hasn't smoked yet, it probably will soon.

[pdf64]

Update:  So the amp works fine with either V3 or V4 removed, think I may have a bad choke…

And as previously requested, please clarify the valve referencing you’re using, as the vibrato channel shouldn’t pass signal with the valve in V4 removed.

[mresistor]

Also if the amp is working fine with a preamp tube removed then how can the choke be bad?

[Danohss]

V3 and V4 are not preamp tubes, they are the reverb circuit which the choke drives.  The voltage is there briefly then drops as the meter is connected for a few seconds.  V3 is on the input side of the reverb circuit, V4 is the driver out.  The preamp tubes are V1 and V2.  The choke is supposed to deliver around 410 Volts to the plate on the input side of the reverb circuit - see photo attached.  That’s where I have around the 410 VDC into the choke after it settles down but only 100ish VDC out to the plate. May be something else wrong as I don’t see how low plate voltage would cause such wicked loud noise, but at the very least, the choke seems to be delivering lower than expected voltage.

[mresistor]

If you pull V3 does the 410V stay stable (V3 socket pin 6) ?

[Keppy]

I'm starting to think that when you say "choke," you're actually referring to the reverb transformer TR4. Is that the case? If so, that's why everyone seems confused. If the actual choke in the power supply was bad, it would affect every non-rectifier tube in the amp.


A bad reverb transformer could certainly affect the plate voltage on the 12AT7, but not the 7025. It seems more likely that you have a bad 12AT7. Did you try replacing that one yet? I only saw in your first post that you replaced the 7025. Any sounds caused by the 12AT7 would be amplified by the 7025, so that would also explain why removing either tube quiets things down.


It would be a good idea to meter the voltages at the grid and cathode of the 12AT7 as well. Those readings would be helpful in case any of the resistors or caps connected to that tube have failed or gone off spec.

[Danohss]

So its odd, having pulled and replaced both tubes a few times, the issue has disappeared.  Thinking arcing on a socket maybe but magnifying glass review shows no evidence of that.  Any chance its was just some bad oxidation on the tube socket contacts?  I’ve never had a problem like that just go away. 

[pdf64]

Yes, something’s certainly odd, ie this thread

[Leevi]

I have had many times issues around the v4.
One problem is that the preamp part of the V4 uses a shared cathode with the reverb recovery stage. The grid of the reverb recovery is grounded at the RCA socket (tank output) with 220K grid leak resistor and 3rd preamp stage grid and the cathode are grounded at the preamp ground bus. That causes often noise especially if the reverb is opened.


I have used successfully the following solution:


Change at least the RCA socket (tank output) connected to the recovery grid to an isolated one. Ground the 220K grid leak resistor and the recovery cathode to the preamp bus. Use a shielded wire from the RCA socket to the grid if the distance is long.


/Leevi

[Danohss]

Problem not gone, what I have is still intermittent loud static, volume pot and reverb pot have no effect.  Valves 3 and 4 pulled amp works fine (obviously no reverb).  Wondering if there may be something that settles down the longer I have the amp powered up?  All bypass caps were replaced with Spragues a few years back, if it was the last filter cap it would also be effecting the preamp tubes so not likely a filter cap issue.  It sat for 10 days and the problem is back, the longer I keep it on and poke around (looking for something loose) the less of the problem I have - static become more intermittent.  I have put 2 new tubes in V3 and 4, does not correct the problem. 

[Danohss]

Latest - I think I’ve narrowed it down to the Reverb tranny - here’s my logic:  With either V3 or V4 pulled the noise goes away.  If it was anything on the V4 side (reverb recovery) the noise would still be present so logic says its somewhere in the V3 (12AT7) circuit.  I’ve replaced the cathode bypass cap and 2.2K resistor as well as the 1M, no change.  When troubleshooting tonight I found that of I put my meter on pins 1 or 6 (blue lead from the reverb tranny) measuring DC voltage to ground problem goes away. I have put 2 different new 12AT7’s in, no change so its not a bad valve. Does that sound logical or is there something else that could be at play here?  Doubt its a bad socket as the meter shouldn’t have any effect on a bad tube socket. 

[PRR]

>   Doubt its a bad socket as the meter shouldn’t have any effect on a bad tube socket. 

Makes as much sense as meter on the transformer. Poking a pin-terminal may strain a bad contact into working better. I've gotten through shows with a wad of coasters wedged between two plugs.

But whatever works!! Buy a cheap meter and leave it inside the amp. I might have left a 'scope on a rig for concert because we know it never misbehaves if you can see it happen.

A 1Meg 1/2W resistor or a 200pFd 500V cap pins 1&6 to ground might be a cheap equivalent meter.

I usually resist replacing iron. It is almost never the problem, and shipping is costly. But the reverb tranny isn't that big, and may be super-fine (fragile) wire.